The present invention relates to a method and system for combusting fuel that has direct application to heat consuming devices such as boilers and furnaces as well as reactors that utilize separated oxygen. More particularly, the present invention relates to such a combustion method and system in which combustion is enhanced with oxygen produced by the use of a ceramic membrane system. Even more particularly, the present invention relates to such a method and system in which the ceramic membrane system is subjected to a countercurrent reactive purge or flow of sweep gas.
Growing concerns about environmental issues, such as global warming and pollutant emissions, are driving industries to explore new ways to increase efficiency and reduce emissions of pollutants. This is particularly true for fossil fuel fired combustion systems, which represent one of the largest sources of carbon dioxide and air pollution emissions. One effective way to reduce emissions and to increase efficiency is to use oxygen, or oxygen enriched air, in the combustion process. The use of oxygen or oxygen enriched air reduces stack heat losses, which increases the system efficiency, while at the same time reducing NOx emissions. Further, the concentration of carbon dioxide in the flue gas is higher since there is little or no nitrogen to act as a diluent. The higher carbon dioxide concentration enhances carbon dioxide recovery options.
Oxygen using the prior art has been limited to those processes with high exhaust temperatures, such as glass furnaces. In such applications, the fuel savings and other benefits achieved are greater than the cost of the oxygen. In low exhaust temperature systems, such as boilers, the reverse is true. In these systems, the cost of oxygen produced with current technologies is more expensive than the available fuel savings. This makes oxygen use in such systems economically unattractive. Moreover, when the energy required to produce the oxygen is taken into consideration, the overall thermal efficiency decreases.
Oxygen transport membranes have been advantageously utilized in the prior art to produce oxygen for heat consuming devices and processes in a manner that results in a savings of energy that would otherwise have to be expended in the separation of oxygen. Oxygen transport membranes are fabricated from oxygen-selective, ion transport ceramics in the form of tubes or plates that are in themselves impervious to the flow of oxygen. Such ceramics, however, exhibit infinite oxygen selectivity at high temperatures by transporting oxygen ions through the membrane. In oxygen transport membranes, the oxygen is ionized on one surface of the membrane to form oxygen ions that are transported through the membrane. The oxygen ions on the opposite side of the membrane recombine to form oxygen with the production of electrons. Depending upon the type of ceramic, oxygen ions either flow through the membrane to ionize the oxygen or along separate electrical pathways within the membrane, or by an applied electric potential. Such solid electrolyte membranes are made from inorganic oxides, typified by calcium-or yttrium-stabilized zirconium and analogous oxides having fluoride or perovskite structures.
In U.S. Pat. No. 5,888,272 oxygen transport membranes are integrated into a combustion process itself, with all the oxygen produced going directly into the combustor. The heated flue gases can then be routed to a heat consuming process. In one embodiment, flue gases are recycled through a bank of oxygen transport membrane tubes and enriched with oxygen. Typically the flue gas enters the bank containing anywhere from 1 to about 3 percent oxygen and leaves the bank containing from about 10 to about 30 percent oxygen by volume. The enriched flue gas is then sent to a combustion space where it is used to burn fuel. In another embodiment, called reactive purge, the oxygen transport membrane tubes are placed directly in the combustion space. A fuel and flue gas mixture, is passed through the tubes and combust with the oxygen as it passes through the tubes. Thus oxygen production and combustion take place simultaneously inside the oxygen transport membrane with the fuel diluted with flue gas.
As will be discussed, the present invention utilizes oxygen transport membranes to produce oxygen to support combustion that inherently reduces the energy expenditures involved in compressing an incoming oxygen containing feed to the membranes. Combustion can take place at the surface of the oxygen transport membranes in the presence of fuel that is not diluted with flue gas.
The present invention provides methods and systems for combusting fuel that have direct application to such heat consuming devices as boilers and furnaces or to reactors that separate oxygen from an oxygen-containing feed. Such reactors include devices for separating oxygen to produce a nitrogen-enriched product.
In accordance with one method of the present invention, an oxygen-containing stream is introduced into at least one oxygen transport membrane. The membrane projects into a combustion zone to separate oxygen from the oxygen-containing stream and thereby, to introduce an oxygen permeate into the combustion zone. A fuel stream is introduced into the combustion zone and fuel within the fuel stream is combusted in the presence of the oxygen permeate so that the at least one oxygen transport membrane is subjected to a reactive purge and a portion of heat arising from the combustion of the fuel heats the at least one ceramic membrane to an operational temperature. Radiant heat energy emanating from the at least one oxygen transport membrane is absorbed within a heat sink to promote stabilization of the operational temperature of the at least one oxygen transport membrane.
The at least one oxygen transport membrane can comprise at least one row of oxygen transport membranes spaced apart from one another. The fuel stream is introduced in a cross-flow relationship to the at least one row of oxygen transport membranes.
It is to be noted that the term, xe2x80x9ccross-flowxe2x80x9d as used herein and in the claims means a flow direction with respect to the oxygen transport membranes that is at right angles to the length of the oxygen transport membranes plus or minus about forty-five degrees. For instance, if tubular oxygen transport membranes are used, the xe2x80x9ccross-flowxe2x80x9d direction would be at or near right angles to the tube as opposed to a direction parallel to the length of the tube as measured between its ends. As such, in xe2x80x9ccross-flowxe2x80x9d the fuel stream and therefore, the reactive purge, can be directed anywhere from an angle directly in line with the row to a direction at right angles to the row. Furthermore, the term xe2x80x9crowxe2x80x9d as used herein and the claims means any arrangement of oxygen transport membranes in a single file. The oxygen transport membranes to be in a xe2x80x9crowxe2x80x9d do not necessarily, however, have to be positioned so that one oxygen transport membrane is directly in front of or behind another oxygen transport membrane. For instance, oxygen transport membranes may be staggered so that each membrane has full benefit of the reactive purge, or as will be discussed, a sweep gas such that each oxygen transport membrane can take full advantage of such a reactive purge or sweep gas acting at least substantially parallel to the line of oxygen transport membranes making up a row.
It should be pointed out that a cross-flow arrangement is advantageous over flow arrangements that act parallel to the length of the oxygen transport membranes. One major advantage is that all adjacent oxygen transport membranes, as viewed in a transverse direction to the reactive purge will see the same combustion conditions. Furthermore, the fuel composition will be substantially the same from the top to the bottom of an oxygen transport membrane. This will promote uniformity in the oxygen flux and therefore, the combustion flux for the reactive purge along the length of an oxygen transport membrane. Since, the composition of the surrounding gas will change as one moves from such transverse sets of oxygen transport membranes it is conceivable that different materials could be advantageously used in subsequent sets of oxygen transport membranes. Furthermore, the rows might be designed to provide additional transverse sets of such adjacent oxygen transport membranes that would provide a back-up upon the degradation of a preceding transverse set of oxygen transport membranes.
The heat sink with respect to the at least one row of oxygen transport membranes can comprise tubes of flowing heat absorbing fluid interspersed within the at least one row of oxygen transport membranes. The tubes of flowing heat absorbing fluid can be steam tubes to heat water flowing therein. In such case, the method of the present invention would be applied to a boiler.
The at least one row of oxygen transport membranes can be connected in series to produce a flow path of retentate streams passing to successive oxygen transport membranes having ever more lean oxygen concentrations. The fuel stream can be introduced into the combustion zone in a counter-current flow direction as viewed with respect to the flow path of the retentate streams so that the reactive purge acts in the counter-current flow direction.
In accordance with another method of the present invention, at least one oxygen transport membrane projects into a separation zone to separate oxygen from the oxygen-containing stream and thereby, to introduce the oxygen permeate into the separation zone. The at least one oxygen transport membrane is heated to an operational temperature. A fuel stream is combusted in a combustion zone located within the heat consuming device to produce a flue gas stream. A sweep gas stream composed of part of the flue gas stream is circulated within the separation zone. Further, the sweep gas stream is circulated from the separation zone to the combustion zone to support combustion of the fuel stream.
The at least one oxygen transport membrane can comprise at least one row of oxygen transport membranes spaced apart from one another and the sweep gas stream can be introduced in a cross-flow relationship to the at least one row of oxygen transport membranes. The oxygen transport membranes can be connected in series to produce a flow path of retentate streams passing to successive oxygen transport membranes and having ever more lean oxygen concentrations. In such case, the sweep gas stream can be circulated in a counter-current flow direction as viewed with respect to the flow path of the retentate streams. The oxygen transport membranes can be heated to the operational temperature by the sweep gas stream.
Advantageously, the sweep gas stream can be circulated by cooling a remaining part of the flue gas stream and injecting the remaining part of the flue gas stream into the separation zone in the form of at least one jet. Alternatively, the sweep gas stream can be circulated by cooling the sweep gas stream after passage through the separation zone and injecting the sweep gas stream into the combustion zone by a blower.
The foregoing method could be used to separate oxygen from air. In such case, the oxygen-enriched stream is air and separation of the oxygen from the oxygen-enriched stream produces a nitrogen-enriched stream. The nitrogen enriched stream can be extracted as a product stream.
The present invention also provides oxygen-enhanced combustion systems that again have principal applications to heat consuming devices and various types of reactors. In one such system, at least one oxygen transport membrane is located within a combustion zone to separate oxygen from an oxygen-containing stream introduced into the at least one oxygen transport membrane, thereby to produce an oxygen permeate. At least one fuel nozzle is provided for injecting a fuel stream of the fuel into the combustion zone so that the at least one oxygen transport membrane is subjected to a reactive purge produced by combustion of the fuel in the presence of the permeated oxygen and a portion of heat arising from the combustion of the fuel heats the at least one ceramic membrane to an operational temperature. A heat sink is positioned to absorb radiant heat energy emanating from the at least one oxygen transport membrane to promote stabilization of the operational temperature thereof.
The at least one oxygen transport membrane can comprise at least one row of oxygen transport membranes spaced apart from one another. The heat sink can comprise tubes of flowing heat absorbing fluid interspersed within the at least one row of oxygen transport membranes. The tubes of flowing heat absorbing fluid can be steam tubes to heat water flowing therein. In such case, the heat consuming device to which the present invention would be applied could be a boiler.
The at least one row of oxygen transport membranes can be connected in series to produce a flow path of retentate streams passing to successive oxygen transport membranes having ever more lean oxygen concentrations. The at least one fuel nozzle can be positioned to introduce the fuel stream into the combustion zone in a counter-current flow direction as viewed with respect to the flow path of the retentate streams so that the reactive purge acts in the counter-current flow direction.
In an alternative system in accordance with the present invention, at least one oxygen transport membrane is positioned within a separation zone of the heat consuming device to introduce the permeated oxygen into the separation zone. At least one nozzle is provided for injecting a fuel stream into a combustion zone for combustion of the fuel stream to produce a flue gas stream. A means is provided for heating the at least one oxygen transport membrane to an operational temperature. A means is also provided for circulating a sweep gas stream composed of a part of the flue gas stream into the separation zone and from the separation zone to the combustion zone to support combustion of the fuel stream. As in other embodiments, the at least one oxygen transport membrane can comprise at least one row of oxygen transport membranes spaced apart from one another. The sweep gas circulation means circulate the sweep gas stream in a cross-flow relationship to the at least one row of oxygen transport membranes. The oxygen transport membranes can be connected in series to produce a flow path of retentate streams passing to successive oxygen transport membranes and having ever more lean oxygen concentrations. In such case, the sweep gas stream is circulated in a counter-current flow direction as viewed with respect to the flow path of the retentate streams. The heating means can comprise heat transfer from the sweep gas stream to the oxygen transport membranes. The foregoing aspects of the present invention could be applied to a furnace or a boiler.
The circulation means can include a heat exchanger to cool a remaining part of the flue gas stream. Additionally, at least one flue gas nozzle is provided to inject at least one flue gas jet composed of the flue gas stream into the separation zone and a blower interposed between the heat exchanger and the at least one flue gas nozzle. Alternatively, the circulation means can comprise a heat exchanger to cool the sweep gas stream. The heat exchanger is positioned to receive the sweep gas stream after having passed through the separation zone. An inlet to the combustion zone is provided and a blower is interposed between the heat exchanger and the inlet to inject the sweep gas stream into the combustion zone.
In embodiments of the present invention in which the oxygen transport membranes are connected in series, as retentate streams emanating from the oxygen transport membranes are sequentially introduced into the membranes of the row, the oxygen content of the feed to each membrane decreases and therefore the amount of oxygen permeated through each successive membrane also decreases. Thus, the permeated oxygen in the vicinity of the last of the oxygen transport membranes in the row is at a lower concentration and therefore, a lower oxygen partial pressure than at the first of the oxygen transport membranes in the row. At the same time, the oxygen partial pressure within each of the oxygen transport membranes is also successively decreasing as it passes to successive membranes in a row. If the partial pressure of the permeated oxygen remains constant or in fact decreases in the vicinity of successive membranes, the pressure driving force for effecting the separation in such successive oxygen transport membranes is also decreasing.
As a result of the ever decreasing pressure driving force, in successive oxygen transport membranes, in order to effect the separation at the last oxygen transport membrane in the row, the separation needs more facilitation by the reactive purge or sweep gas than at the first of the oxygen transport membranes. This naturally occurs in the present invention due to the countercurrent flow of the fuel stream that can act as a reactive purge or the sweep gas. In case of a reactive purge provided by the fuel stream, as the fuel flows in the counter-current direction, the fuel is consumed and thus, the concentration of fuel within the bulk flow of fuel and combustion gases decreases. As a result, it becomes increasingly difficult for the fuel to diffuse to the surface of the membrane and combust. Therefore, the reactive purge is most effective at the last of the oxygen transport membranes in the row where the greatest facilitation of oxygen separation by the reactive purge is required. As the flow of fuel containing gases flows along the row, diffusion of the fuel to the surface of the membrane is more difficult due to the dilution of fuel within the combustion gases. However, less facilitation is required due to the increasing pressure driving force in successive oxygen transport membranes towards the first of the oxygen transport membranes.
The action of a counter-current flow of sweep gas has a similar effect to the reactive purge in that as it flows in the counter-current direction, it has the lowest concentration of oxygen at the last of the oxygen transport membranes in the row and therefore is most able to facilitate the separation at such oxygen transport membrane. As it travels in the counter-current direction and gains oxygen, it is least able to facilitate the separation. However, less facilitation is required in successive oxygen transport membranes taken in a direction from the last of the oxygen transport membranes to the first of the oxygen transport membranes.
As may be appreciated, the use of any reactive purge reduces the degree of compression for the incoming feed such that only a blower or an induced draft fan might be necessary to circulate the oxygen-containing gas into the oxygen transport membranes. The use of a counter-current reactive purge or sweep gas, reduces the degree of compression that would otherwise be required to compress the feed to an oxygen transport membrane system. This reduction of compressive effort makes the application of the present invention attractive even in low exhaust temperature systems such as boilers.
In the present invention, the reactive purge involves the combustion of fuel in the presence of oxygen separated by the membrane. As a result, this combustion of oxygen takes place at or near the surface of the membrane to produce a driving force for the separation to also lessen or possibly eliminate the degree to which the incoming oxygen containing feed need be compressed. Hence, the reactive purge of the present invention has application to any membrane system whether or not there are multiple membranes used or multiple membranes are connected in series.
Since, the adiabatic flame temperature of ambient temperature methane and pure oxygen exceeds 5000xc2x0 F., direct combustion of natural gas on the surface of an oxygen transport membrane is not normally considered. In the prior art, the excessive temperature problem involved in reactive purging is overcome by mixing a small amount of fuel with a large amount of non-reactive purge gas. In many membrane types, the flux of oxygen through the membrane increases as the membrane temperature increases. The combustion reaction at the surface, and therefore the heat release at the surface, is therefore limited by the oxygen flux through the membrane. However, poor temperature control can lead to catastrophic thermal runaway of the membrane. As the temperature increases more oxygen passes through the membrane leading to higher combustion rates at the surface and still higher membrane temperatures until the temperature limitations of the membrane is exceeded. As will be discussed in more detail, the inventors herein have found that temperature control of the membranes can be accomplished by appropriate placement or arrangement of the membranes with respect to a heat sink that can absorb radiant heat and therefore prevent damaging thermal runaway.